Two antibiotics have been found which inhibit both yeast RNA polymerases I and II in vitro by interaction with the enzymes rather than with the template. These antibiotics inhibit yeast cultures at similar concentrations, suggesting that the nuclear transcription apparatus is their target in vivo. Their mode of action will be determined by investigating their effects on initiation and progression of RNA synthesis on natural (yeast), native, denatured and synthetic templates. The kinetics of inhibition of synthesis of different RNA classes will be studied in permeabilized cells, spheroplasts and nuclei. Antibiotic-resistant mutants with resistant RNA polymerases will be sought, the nature of the alterations investigated and the mutations mapped. The mutants will be screened for conditional defects in growth and defects in non-obligatory events involving differentiation of cellular function: mating, meiosis and sporulation. Related studies in other sensitive eukaryotic microorganisms will be initiated.